North Korea's parliament has endorsed plans to give nuclear weapons greater prominence in the country's defences.
The move came a day after the ruling Workers' Party called for nuclear forces to be "expanded and beefed up qualitatively and quantitatively".
North Korea has said it is entering a "state of war" with the South, prompting Seoul to promise a strong and swift military response to any provocation from the North without regard to political consequences.
The North is angry at UN sanctions following its nuclear test in February and is unhappy with joint US-South Korea annual military drills, the BBC reports.
On Sunday the Workers' Party Central Committee held a rare high-level meeting in which it described nuclear weapons as "the nation's life".
South Korean president Park Geun-hye says she takes the recent threats very seriously.
In the last few days North Korea has issued multiple warnings of attacks on US and South Korean targets - to which the US has responded with an apparent show of military hardware.
Officials said the US flew F-22 planes from Japan to South Korea's Osan Air base on Sunday, as part of ongoing joint military exercises with Seoul, officials said.